The present invention relates to a magnetic recording medium and a process for producing the magnetic recording medium, and more particularly, to a magnetic recording medium capable of exhibiting a relatively high coercive force, especially a coercive force of not less than 159 kA/m (2,000 Oe) despite as small a film thickness as not more than 200 nm, and an excellent surface smoothness, and a process for producing the magnetic recording medium at one step.
In recent years, in magnetic recording apparatuses such as hard disk devices, there has been a remarkable tendency that information devices or systems used therefor are miniaturized and required to have a high reliability. With such a recent tendency, in order to deal with a large capacity data, there is an increasing demand for providing magnetic recording media on which information can be stored with a high density.
In order to satisfy such requirements, the magnetic recording media have been strongly required to have not only a high coercive force, but also reduce a distance between a magnetic head and a magnetic recording layer (magnetic spacing).
As magnetic recording media having a high coercive force, there is widely known those comprising a substrate and a magnetic thin film formed on the substrate.
The magnetic thin films which have been already practically used in magnetic recording media, are generally classified into magnetic oxide thin films composed of iron oxides such as magnetite, maghemite, etc. (xe2x80x9cTechnical Report of the Institute of Electronics, Information and Communication Engineersxe2x80x9d, published by the of Electronics, Information and Communication Engineers, (1981) MR81-20, pp. 5 to 12; xe2x80x9cCeramicsxe2x80x9d, published by Japan Institute of Ceramics, (1986) Vol. 24, No. 1, pp. 21 to 24; and Japanese Patent Publication (KOKOKU) Nos. 51-4086(1976) and 5-63925(1993)), and magnetic alloy thin films composed of Coxe2x80x94Cr alloy or the like.
The magnetic oxide thin films composed of iron oxides are excellent in corrosion resistance due to inherent properties of the oxides. As a result, the magnetic oxide thin films can show an excellent stability independent of change with passage of time, and less change in magnetic properties with passage of time. Further, since oxides exhibit a higher hardness than that of metals, it is not necessary to form an overcoat thereon. As a result, it becomes possible to reduce the magnetic spacing of the obtained magnetic recording medium as compared to those magnetic recording media using the magnetic alloy thin film. Therefore, the magnetic oxide thin film is optimum for the production of high-density magnetic recording media.
It has been attempted to enhance a coercive force of the thin iron oxide film by incorporating cobalt thereinto. However, with the increase in cobalt content, the iron oxide thin film tends to be deteriorated in stability independent of change with passage of time due to adverse influences of heat or the like.
Meanwhile, the present inventors have already proposed a maghemite thin film which can exhibits a high coercive force even with a less cobalt content by controlling the spacing of specific plane of the maghemite thin film (Japanese Patent Application Laid-Open (KOKAI) Nos. 11-110731(1999) and 11-110732(1999)).
On the other hand, the magnetic alloy thin films composed of Coxe2x80x94Cr alloy or the like, have a coercive force as high as not less than about 159 kA/m (2,000 Oe). However, these alloy materials tend to be readily oxidized in themselves and, therefore, tend to be deteriorated instability independent of change with passage of time as well as magnetic properties.
In order to prevent the deterioration of magnetic properties due to the oxidation, a overcoat composed of diamond-like carbon, SiO2 or the like, having a thickness of usually about 10 to 20 nm, is formed on the surface of the magnetic alloy thin film, resulting in undesired increase in magnetic spacing corresponding to the thickness of the overcoat.
In magnetic recording media, in order to reduce the magnetic spacing, it is necessary to reduce the flying height of a magnetic head as far as possible, and always cause the magnetic head to be flying stably. In conventional hard disk drive, magnetic recording media used therefor have been required to have a certain surface roughness in order to prevent the magnetic head from being absorbed thereonto owing to a meniscus force therebetween upon stopping the magnetic head. At the present time, as a result of current improvement in these hard disk systems, magnetic recording media have been no longer required to show such a surface roughness for preventing the magnetic head from being absorbed thereonto. On the contrary, it has been required that a magnetic thin film used in these magnetic recording media exhibits a more excellent surface smoothness.
Also, it is known that the non-smooth surface of magnetic recording media causes media noise. In order to eliminate such a media noise, it is necessary to reduce a surface roughness of the magnetic thin film.
At present, in magnetic recording media using a magnetic oxide thin film, with the recent tendency toward high recording density, the magnetic oxide thin film is required to have a very small thickness such as not more than 200 nm. For this reason, the surface properties of the magnetic thin film are considerably influenced by surface properties of a substrate. As a result, it has been required not only to use such a substrate having an excellent surface smoothness, but also to develop techniques for further smoothening the surface of the magnetic thin film.
Hitherto, as methods of producing iron oxide thin films, there are known (1) a method of forming a magnetite thin film on a substrate, and then oxidizing the magnetite thin film at a temperature of not less than 300xc2x0 C.; (2) a method of forming a film composed of xcex3-Fe2O3, Fe3O4 or FeOx (4/3 less than x less than 3/2) using xcex3-Fe2O3, Fe3O4 or FeOx (4/3 less than x less than 3/2) as a target (Japanese Patent Publication (KOKOKU) Nos. 62-49724(1987) and 6-61130(1994)); (3) a method of forming a cobalt-containing spinel-type iron oxide thin film directly on a substrate by sputtering a target while irradiating the substrate with a high-density oxygen plasma (Japanese Patent Application Laid-Open (KOKAI) Nos. 1-298029(1989) and 3-78114 (1991)); or the like.
Presently, it has been strongly demanded to provide magnetic recording media comprising a spinel-type iron oxide thin film capable of showing a high coercive force and an excellent surface smoothness despite its small thickness. However, such magnetic recording media satisfying these requirements have not been obtained until now.
Namely, in the above method (1) for producing the iron oxide thin film, the obtained magnetite thin film is taken out into atmosphere and further subjected to oxidation treatment at a temperature of 300 to 450xc2x0 C., thereby obtaining the maghemite thin film. In this method, since it is necessary to conduct the heat-treatment at a temperature as high as not less than 300xc2x0 C., there arises such a problem that the obtained thin film is deteriorated in magnetic properties due to migration from the substrate or the like. Therefore, it is required to select the material of the substrate from those having an excellent heat resistance, so that only limited substrates are usable therefor. In addition, since the magnetite thin film is taken out into atmosphere, there also arise problems such as contamination thereof.
In the method (2), the obtained iron oxide thin film shows a poor coercive force and, therefore, unsatisfactory in magnetic properties.
In the method (3) for producing the iron oxide thin film as described in Japanese Patent Application Laid-Open (KOKAI) No. 1-298029(1989), the spinel-type iron oxide thin film is formed by irradiating the substrate with a high-density oxygen plasma generated from an ECR plasma device while sputtering each metal target. The obtained thin film exhibits a coercive force as high as 191 kA/m (2,400 Oe), but has a very large thickness of 750 nm. Further, since the spinel-type iron oxide thin film is intended for use in magneto-optical recording media, the surface smoothness thereof is not taken into consideration. Therefore, the spinel-type iron oxide thin film obtained by the method (3) is unsatisfactory in aimed surface smoothness. Further, in Japanese Patent Application Laid-Open (KOKAI) No. 3-78114(1991), it is described that iron oxide is produced by sputtering an oxide target while irradiating the oxide target with ECR plasma. However, the obtained iron oxide shows a low coercive force and, therefore, is unsatisfactory in magnetic properties.
As a result of the present inventors"" earnest studies for solving the above problems, it has been found that by sputtering a metal target or a metal alloy target in an atmosphere of plasma activated by electron cyclotron resonance (ECR) microwave, the obtained Co-containing spinel-type iron oxide thin film can exhibit a Co content of 1 to 20 mol % based on Fe, a coercive force of not less than 159 kA/m (2,000 Oe), a film thickness of 5 to 200 nm, a center line average height (Ra) of 0.1 to 0.8 nm and a maximum height (Rmax) of not more than 10 nm. The present invention has been attained based on the above finding.
An object of the present invention is to provide a magnetic recording medium capable of exhibiting a coercive force of not less than 159 kA/m (2,000 Oe), a film thickness of 5 to 200 nm, a center line average height (Ra) of 0.1 to 0.8 nm and a maximum height (Rmax) of not more than 10 nm.
Another object of the present invention is to provide a process for producing a magnetic recording medium having a relatively high coercive force, especially not less than 159 kA/m (2,000 Oe) despite as small a film thickness as not more than 200 nm, and an excellent surface smoothness, only at one step.
To accomplish the aims, in a first aspect of the present invention, there is provided a magnetic recording medium, comprising:
a substrate; and
a Co-containing spinel-type iron oxide thin film formed on the substrate, having a Co content of 1 to 20 mol % based on Fe, a coercive force value of not less than 159 kA/m (2,000 Oe), a thickness of 5 to 200 nm, a center line average height Ra of 0.1 to 0.8 nm and a maximum height (Rmax) of not more than 10 nm.
In a second aspect of the present invention, there is provided a magnetic recording medium, comprising:
a substrate; and
a Co-containing spinel-type iron oxide thin film formed on the substrate, comprising magnetite represented by the general formula of FeOxxc2x7Fe2O3 (0 less than xxe2x89xa61) and maghemite represented by the general formula of xcex3-Fe2O3, and having a Co content of 1 to 20 mol % based on Fe, a coercive force value of not less than 159 kA/m (2,000 Oe), a thickness of 5 to 200 nm, a center line average height Ra of 0.1 to 0.8 nm, a maximum height (Rmax) of not more than 10 nm, a surface electrical resistance value of not more than 1.5 Mxcexa9 and a saturation magnetization value of 29 to 63 Wb/m3 when measured by applying a magnetic field of 1,590 kA/m (20 kOe) thereto.
In a third aspect of the present invention, there is provided a magnetic recording medium, comprising:
a substrate;
an underlayer formed between the substrate and the Co-containing spinel-type iron oxide thin film, said underlayer having a thickness of not more than 200 nm and being ones selected from the group consisting of an oxide thin film having a NaCl-type structure, a metal thin film having a bcc structure, a metal thin film having a B2 structure and an oxide thin film exhibiting an amorphous structure by X-ray analysis; and
a Co-containing spinel-type iron oxide thin film formed on the underlayer, having a Co content of 1 to 20 mol % based on Fe, a coercive force value of not less than 159 kA/m (2,000 Oe), a thickness of 5 to 200 nm, a center line average height Ra of 0.1 to 0.8 nm and a maximum height (Rmax) of not more than 10 nm.
In a fourth aspect of the present invention, there is provided a process for producing a magnetic recording medium, comprising:
forming a Co-containing iron oxide thin film on a substrate by sputtering a metal target or a metal alloy target in an atmosphere of plasma activated by an electron cyclotron resonance microwave.